diff --git a/src/librustc/middle/dependency_format.rs b/src/librustc/middle/dependency_format.rs
index e7055827c491fd6eb74f8252e118c07cc566aac2..f7dfc3c33c0a7830f2b14fac2e03b1ba4519c49f 100644
--- a/src/librustc/middle/dependency_format.rs
+++ b/src/librustc/middle/dependency_format.rs
@@ -94,13 +94,14 @@ pub enum Linkage {
 
 pub fn calculate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     let sess = &tcx.sess;
-    let mut fmts = sess.dependency_formats.borrow_mut();
+    let mut fmts = FxHashMap();
     for &ty in sess.crate_types.borrow().iter() {
         let linkage = calculate_type(tcx, ty);
         verify_ok(tcx, &linkage);
         fmts.insert(ty, linkage);
     }
     sess.abort_if_errors();
+    sess.dependency_formats.set(fmts);
 }
 
 fn calculate_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index 499c45e28fe804574301f0aa391563386ebea5f0..55da5b921af0064ce0745ce3fb0d2c0b98815af2 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -22,7 +22,7 @@
 use session::search_paths::PathKind;
 use session::config::{DebugInfoLevel, OutputType};
 use ty::tls;
-use util::nodemap::{FxHashMap, FxHashSet};
+use util::nodemap::{FxHashSet};
 use util::common::{duration_to_secs_str, ErrorReported};
 use util::common::ProfileQueriesMsg;
 
@@ -93,7 +93,7 @@ pub struct Session {
     pub plugin_llvm_passes: OneThread<RefCell<Vec<String>>>,
     pub plugin_attributes: OneThread<RefCell<Vec<(String, AttributeType)>>>,
     pub crate_types: Once<Vec<config::CrateType>>,
-    pub dependency_formats: RefCell<dependency_format::Dependencies>,
+    pub dependency_formats: Once<dependency_format::Dependencies>,
     /// The crate_disambiguator is constructed out of all the `-C metadata`
     /// arguments passed to the compiler. Its value together with the crate-name
     /// forms a unique global identifier for the crate. It is used to allow
@@ -1097,7 +1097,7 @@ pub fn build_session_(
         plugin_llvm_passes: OneThread::new(RefCell::new(Vec::new())),
         plugin_attributes: OneThread::new(RefCell::new(Vec::new())),
         crate_types: Once::new(),
-        dependency_formats: RefCell::new(FxHashMap()),
+        dependency_formats: Once::new(),
         crate_disambiguator: Once::new(),
         features: Once::new(),
         recursion_limit: Once::new(),